xmss: add cross test with ream signature

[tcoratger]

🗒️ Description

The test is generated in Ream using:

#[test]
fn test_cross_client_compat_vectors() {
    let mut rng = ChaCha20Rng::seed_from_u64(42);

    let activation_epoch = 0;
    let num_active_epochs = 10;

    let (public_key, private_key) =
        PrivateKey::generate_key_pair(&mut rng, activation_epoch, num_active_epochs);

    let epoch = 5;
    let message = [0u8; 32];

    let signature = private_key
        .sign(&message, epoch)
        .expect("Signing should succeed");

    // Verify in ream first
    let is_valid = signature
        .verify(&public_key, epoch, &message)
        .expect("Verification should not error");
    assert!(is_valid, "Signature must verify in ream");

    // Assert expected sizes
    assert_eq!(public_key.inner.len(), 52, "PublicKey must be 52 bytes");
    assert_eq!(signature.inner.len(), 3112, "Signature must be 3112 bytes");

    // Print hex vectors for the Python test suite
    println!("PUBLIC_KEY_HEX={}", hex::encode(public_key.inner));
    println!("SIGNATURE_HEX={}", hex::encode(signature.inner));
    println!("EPOCH={epoch}");
    println!("MESSAGE={}", hex::encode(message));
}

🔗 Related Issues or PRs

✅ Checklist

• Ran tox checks to avoid unnecessary CI fails:

  uvx tox

• Considered adding appropriate tests for the changes.
• Considered updating the online docs in the ./docs/ directory.